Proton exchange membrane fuel cell power plants are known in the art and have shown great promise as an energy source for the future. The use of such fuel cells has become increasingly important for space flight, both manned and unmanned, to supply power over extended periods of time without the need for often unreliable sources of energy such as solar cells. It is also expected that such fuel cell systems will soon find broad application to terrestrial systems, as well.
In application of fuel cell systems to space flight, a problem exists with the separation of fuel cell product water from circulating oxidant gas in variable gravity conditions. In addition, solutions to this problem must use a minimum of power while providing for safe, reliable, and maintenance-free operation of the device for dealing with product water.
A number of water separators have been suggested in the art for fuel cell systems under normal gravity conditions. Such systems have included motor-driven centrifugal separators, including vortex-type separators; gravity fed separators; certain wicking schemes; bubble-point pressure control for water outlet flow control; and hydrophilic/hydrophobic permeable membrane geometries to accomplish product water separation from a circulating reactant gas stream. However, these known systems either require too much power, are unreliable, are difficult to maintain, or exhibit all of these drawbacks. Further, water separator systems specifically designed for operation on the surface of the earth may not operate efficiently, or even at all, when confronted with conditions of varying gravity.
For example, as described in U.S. Pat. No. 5,503,944 to Meyer et al., a known problem in the operation of solid polymer, or proton exchange membrane, fuel cells relates to the management of water, both coolant and product water in the cells of the power plant. In a proton exchange membrane fuel cell, product water is formed by the electrochemical reaction at the membrane on the cathode side of the cells by the combination there of hydrogen and oxygen ions. The product water must be drawn away from the cathode side of the cells. However, makeup water must be provided to the anode side of the cells in amounts sufficient to prevent dry out while avoiding flooding of the anode side of the electrolyte membrane.
To address this need in the art, the system shown and described in the Meyer et al. '944 patent uses porous plates physically close to the cathode reaction sites of a fuel cell, whereby the product water produced at the cathode reaction sites travels through the porous plates to the coolant water stream. This transfer is caused by a pressure differential maintained between the oxygen and the coolant water streams. Excess water is removed from the coolant water stream. Control of water from the cathode reaction sites to coolant water passages is accomplished with the use of the “bubble-point” pressure of the porous plates. However, there is no active control of water outlet flow rates, and the use of bubble-point pressures and porous plates is affected by the thickness of the plate, as is the structural capability of the plates. This particular design solution is also difficult to configure initially when filling all the pores of all the plates uniformly with water. Furthermore, it is difficult to recover from a “blow through” condition, which is loss of the water seal in the porous plates due to exceeding the bubble point pressure.
U.S. Pat. No. 6,579,637 to Savage et al. purports to provide a fuel cell system having a compact, efficient, low-pressure-drop water separator for removing liquid water droplets from water-laden system streams. Water separators are necessary at several points within this system. The system described provides a swirling feature that provides for centrifugal separation of water droplets with the use of fixed swirl vanes. The separated water is then driven by the general fluid flow toward a gravity-dependent, float-switch-controlled water outlet valve. Such a system could not function in an application in which zero gravity and high gravity conditions may be variously encountered.
A water separator for use in an earth-bound vehicle is shown and described in U.S. Pat. No. 6,485,854 to Grover et al. The separator includes a variable flow restriction device to be used as an oxygen and water separator. The variable flow restriction is intended to be adjusted to match the water separation needs of the fuel cell system in which it would be used. As with previously described systems, this system relies on gravity for functions of the system, and thus is inappropriate for variable gravity applications.
Thus, there remains a need for a simple, reliable, energy efficient water separator for use in a fuel cell system which must operate in zero gravity or multi-G conditions. The present invention addresses this need in the art.